So like there is this volcano near where I live and i was thinking of flying over it with fpv, since its kinda active I'm thinking its got heat coming out of it. Do you think spraying the bottom of my seaplane with fire retardant would shield it from any heat waves it may encounter?

If you're going to fly over an active volcano, no amount of treatment is going to preserve the integrity of the foam, or possibly the electrics.
Do you have foam safe, spray on fire retardant in Iceland

Foam retardant is to prevent a surface from combusting. It will not do much to prevent heat rise. Active magma radiates a lot of heat. You have to assume that your plane will absorb 50-90% of the energy that reaches it. You are in a thermal balance between the heat flux warming the plane and the air around it cooling the plane. You need to figure out the thermal balance point where the energy in equals the energy out. This not a simple problem to figure out.

I don't know how close to the volcano you want to be but higher will obviously keep you cooler.

I was in Iceland this summer, fanastic scenery, which one are you planning to fly over, Eyjafjallajokull?

Aluminum foil tape (for duct-work) will radiate most of the heat (at a weight penalty), but you might have to mount your antenna external to keep it from being shielded.

Any kind of Monokote will shrink in the heat, warping your plane.

Aside from protecting the outside of the plane, your electronics' temperatures will climb quickly. Battery, motor and ESC could overheat unless you have some way to cool them.

It might be a matter of strategy... gain your altitude in cold air, then throttle down and glide over the volcano, catching thermals and updrafts as you go... so the motor is not running while you are in the heat.